1. Field of the Invention
The present invention relates to fiber lasers, and more specifically, it relates to cladding-pumped Raman fiber lasers.
2. Description of Related Art
Many scientific and industrial applications require laser sources that generate mJ-class, 1-30 ns pulses with diffraction limited transverse mode quality. Q-switched oscillators and the master oscillator power amplifier (MOPA) based on crystalline and ceramic gain media are the two traditional approaches to meeting these laser requirements. Since surface damage is one of the key limits to pulse energy scaling, the aperture of these bulk crystals is increased to ensure that the fluence of the pulses remains below damage levels. With careful design, these systems can be engineered to operate close to diffraction limited beam quality. However, maintaining good beam quality as the system is scaled in average power remains a challenge.
On the other hand, fiber lasers and amplifiers can guide a single transverse mode and can potentially obviate the need to make tradeoffs between beam quality and output power. To achieve high pulse energy output from fibers, the core diameter needs to be increased while reducing the numerical aperture (NA) to ensure that only a single transverse mode is guided. Achieving this in traditional large mode area (LMA) fibers poses a manufacturing challenge as very tight tolerances on the index of refraction across the core diameter are required. Photonic crystal (PC) fibers are an alternative means to scale the core diameter while keeping a small core NA. LMA or PC fibers with large core-diameters and long lengths must be bent in order to achieve a compact setup. Bending the fibers causes losses for the higher order modes, reduces the mode field diameter, and therefore reduces the extractable pulse energy. Further, pulse energy scaling with good beam quality has been difficult because long fiber lengths, combined with tightly confined modes with high peak powers, trigger the onset of nonlinear effects like stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS).